1. Field of the Invention
The present invention relates generally to a dual FET differential voltage controlled attenuator, and more particularly to a dual FET differential voltage controlled attenuator having an improved linear attenuation characteristic over a control voltage range.
2. Description of the Background Art
A signal attenuator is a device that, as the name implies, reduces the magnitude and therefore the electrical energy of an incoming waveform signal. The removed electrical energy is dissipated as heat.
Attenuation is commonly used in electrical circuits. For example, signal attenuators are commonly used in audio circuits, radio frequency (RF) circuits, automatic gain control (AGC) circuits, etc. Generally, a waveform signal is initially amplified to a desired maximum level and then attenuated at points where a lower energy level is desired. Attenuation is generally preferred because during amplification of a signal, any noise on the signal is also amplified. It may therefore be desired to amplify a signal only once, and perform any subsequent actions on the amplified signal.
In the prior art, voltage controlled FETs have been commonly used in attenuators in order to attenuate an input signal. The prior art has applied voltage controlled FETs in a variety of ways, as shown in U.S. Pat. No. 5,745,634 to Garrett et al., U.S. Pat. No. 4,334,195 to Luce, U.S. Pat. No. 4,155,047 to Rubens et al., and U.S. Pat. No. 3,581,222 to Dunwoodie.
However, a common drawback of prior art attenuators is distortion. This distortion may be defined as unwanted changes to a waveform amplitude, especially localized changes, phase shifts, changes in waveform shape, etc. In addition, the prior art attenuators suffer from distortion that varies over a range of control voltage used to set the attenuation level. As a result, the input waveform is changed by the attenuator in undesirable and unpredictable ways.
What is needed, therefore, is an improved attenuator that provides a very linear attenuation over an available control voltage range.